The invention described herein was made in the course of work under contract to the Department of Defense.
1. Field of the Invention
This invention relates to solid explosives. More particularly, this invention relates to a molded low density explosive member of controllable pressure which has been molded into a shaped object having interlocking peripheral means using, as main ingredients, pentaerythritol tetranitrate (PETN) powder and expandable foam polystyrene beads and a method of making same.
2. Description of the Related Art
It is desirable to form a solid explosive material into a predetermined shape to achieve control of the direction of the pressure wave propagated upon detonation of the explosive. It is also desirable to control the amount of pressure generated during detonation. Conventionally, this is done by preselecting a particular explosive material or composition which develops pressure in the desired range. It is also desirable to provide a lightweight explosive material.
Stark in U.S. Pat. Nos. 2,768,072; 2,845,025; and 3,049,454 describes a shaped lightweight explosive having a cellular foam structure. The molded material is formed by dissolving the explosive in a solvent, such as styrene, which is then mixed with a cross-linkable unsaturated polyester together with the proper catalysts and accelerators to promote cross-linking. The mixture is then foamed as it sets up either by introducing compressed air through the mixture or by incorporating into the mixture a chemical blowing agent, such as diazoaminobenzene or toluene diisocyanate.
While the foregoing provides a way of forming a molded explosive material, the use of cross-linkable unsaturated polyester resins is both expensive and time-consuming; and the amount of usable explosive is limited to the amount which will dissolve in the solvent. Furthermore, pentaerythritol tetranitrate (PETN) explosive powder is not compatible with liquid resins due to chemical reaction therebetween which presents unacceptable safety risks.
Thomas U.S. Pat. No. 3,198,677 describes the incorporation of an explosive into a foamed polyurethane structure by dispersing the explosive into the monomers before curing of the polyurethane. However, polyurethane foams are usually open cell type foams which permit penetration of moisture which, in turn, will desensitize some solid explosives such as, for example, pentaerythritol tetranitrate (PETN) powder.
Donaghue et al U.S. Pat. No. 4,151,022 also discloses formation of an explosive composition by mixing together an explosive and components capable of forming a foam. The patentees state that the nature of the explosive component is not narrowly critical, and that while the compositions may include high explosives such as trinitrotoluene, pentaerythritol cyclotrimethylenetrinitramine, and the like, they are not essential ingredients and when present, say in the role of sensitizing agents, the proportion thereof in the composition should be a comparatively minor one.
When the resin materials are liquid, the patentees state that the explosive is conveniently mixed with one of the liquid components prior to forming the foam. When the resin materials are solid, the patentees say that the materials are usually converted to a liquid or pasty form prior to blending with the explosive component. Thus, in the instance of a matrix of polystyrene, a matrix is said to be prepared by heating polystyrene particles--admixed with an explosive component--in the presence of a blowing agent and allowing the blowing agent to penetrate the particles.
Araki et al U.S. Pat. No. 4,408,534 disclose a molded explosive in the shape of a sheet or disk with raised projections to contact adjoining sheets or disks formed into a stack. The raised projections keep adjoining sheets or disks from face to face contact with one another.
It would, however, be desirable to mold a dry mixture of an explosive powder, such as pentaerythritol tetranitrate (PETN) powder, and foamable polystyrene beads having a puffing agent previously incorporated therein in a heated mold into a desired foamed shape without exposure to water or any externally added blowing agent during the molding process.
Moreover, it would be desirable to be able to mold such a mixture into shapes having mating means which permit interlocking of adjoining molded explosive members to permit smooth propagation of the pressure wave.